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Lesson: Chapter 14 — Hydrocarbons and their derivatives

Alkanes 3 of 4

The first five alkanes

Table 14.2 gives the formulas and names of the alkanes with 1 to 5 carbon atoms, and table 14.3 draws their structures.

Molecular formula Name of the alkane
CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane
C5H12 pentane

Petrol is a mixture of alkanes; the alkane present in it most is octane, C8H18. LP gas, another mixture of alkanes, contains mainly propane (C3H8) and butane (C4H10).

Table 14.3 — the structures

The structure of propane: 3 carbon atoms in a chain, each joined by single bonds to hydrogen atoms so that every carbon has four bonds.
The structure of butane: 4 carbon atoms in a chain, each joined by single bonds to hydrogen atoms so that every carbon has four bonds.
The structure of pentane: 5 carbon atoms in a chain, each joined by single bonds to hydrogen atoms so that every carbon has four bonds.

Why the names follow a pattern

Every alkane name ends in -ane, which tells you the compound is an alkane. The first part counts the carbons: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, and on to oct- 8 in octane. The first four prefixes are old names; from five onward they are the Greek numbers you may know from pentagon and octagon. So a name tells you the formula, and a formula tells you the name: pentane has five carbons, so it is C5H2×5+2 = C5H12. The same prefixes turn up later in ethene, whose two carbons are shared with ethane.

Meth, eth, prop, but, pent

The first five alkanes are methane, ethane, propane, butane and pentane. Petrol is mostly octane, C8H18; LP gas is propane and butane.